This description of OWL, the Web Ontology Language
being designed by the W3C Web Ontology Working Group,
contains a high-level abstract syntax for both OWL DL and OWL Lite,
sublanguages of OWL.
A model-theoretic semantics is given to provide a formal meaning for OWL
ontologies written in this abstract syntax.
A model-theoretic semantics in the form of an extension to the RDF
semantics is also given to provide a formal meaning for OWL ontologies
as RDF graphs (OWL Full).
A mapping from the abstract syntax to RDF graphs is given and
the two model theories are shown to have the same consequences on
OWL ontologies that can be written in the abstract syntax.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Candidate Recommendation does not imply
endorsement by the W3C Membership. This is a draft document and may be
updated, replaced or obsoleted by other documents at any time. It is
inappropriate to cite this document as other than "work in
progress".

The design of OWL expressed in earlier versions of these documents
has been widely reviewed and satisfies the Working Group's technical
requirements. The Working Group has addressed
all comments received, making changes as necessary. Changes to
this document since the Last Call
Working Draft are detailed in the change log.

The Working Group now hopes to gather experience from the growing
number of OWL
implementations in order to increase confidence in the language
and meet specific exit criteria.
This CR period will extend until at least 20 September 2003. After
that date, when and if the exit criteria are met, the group intends to
request
Proposed Recommendation status.

Although OWL is essentially stable, later versions of these
documents are expected to contain minor improvements. The test site is likely to
include new, clarifying tests, even during this CR period.
Additionally, the design of OWL depends in part on the design of RDF,
and at this time the relevant RDF specifications are only Working
Drafts. It is therefore possible that unanticipated changes in RDF
may require changes to OWL. This document is current with
respect to RDF Editor's Draft changes made up to 7 August 2003.

One technical
detail concerning structure reuse in
Section 4 (Mapping to RDF Graphs)
has been identified as "at
risk" and subject to change. We expect this change, if made, to
simplify rather than complicate implementations, and since it is a
relaxation of a current restriction, it will not invalidate or change
the meaning of any valid OWL or RDF documents.

1. Introduction (Informative)

This document is one part of the specification of OWL, the Web Ontology
Language.
The OWL Overview
[OWL Overview]
describes each of the different documents in the specification
and how they fit together.

This document
contains several interrelated normative
specifications of the several
styles of OWL, the Web Ontology Language being produced by the
W3C Web Ontology Working Group
(WebOnt).
First, Section 2 contains
a high-level, abstract syntax for both
OWL Lite, a subset of OWL,
and OWL DL, a fuller style of using OWL
but one that still places some
limitations on how OWL ontologies are constructed.
Eliminating these limitations results in the full OWL language, called
OWL Full, which has the same syntax
as RDF.
The normative exchange syntax for OWL is
RDF/XML [RDF Syntax];
the OWL Reference document
[OWL Reference]
shows how the RDF syntax is used in OWL.
A mapping from the OWL abstract syntax to
RDF graphs
[RDF Concepts]
is, however, provided in Section 4.

This document contains two formal semantics for OWL.
One of these semantics, defined in
Section 3,
is a direct, standard model-theoretic semantics for
OWL ontologies written in the abstract syntax.
The other, defined in Section 5,
is a vocabulary extension of the RDF semantics
[RDF MT] that provides semantics
for OWL ontologies in the form of RDF graphs.
Two versions of this second semantics are provided, one that corresponds
more closely to the direct semantics (and is thus a semantics for OWL DL)
and one that can be used in cases where classes need to be treated as
individuals or other situations that cannot be handled in the abstract
syntax (and is thus a semantics for OWL Full). These two versions are
actually very close, only differing in how they divide up the domain of
discourse.

Appendix A
contains a proof that the direct and RDFS-compatible semantics have the same
consequences on OWL ontologies that correspond to abstract OWL
ontologies that separate OWL individuals, OWL classes, OWL properties,
and the RDF, RDFS, and OWL structural vocabulary.
Appendix A
also contains the sketch of a proof that the entailments in the
RDFS-compatible semantics for OWL Full include all the entailments in
the RDFS-compatible semantics for OWL DL.
Finally a few examples of the various concepts defined in the document are
presented in Appendix B.

This document is designed to be read by those interested in the
technical details of OWL. It is not particularly intended for the
casual reader, who should probably first read the OWL Guide
[OWL Guide]. Developers of parsers
and other syntactic tools for
OWL will be particularly interested in Sections
2 and 4.
Developers of reasoners and other semantic tools for OWL will be
particulary interested in Sections
3 and 5.

Appendix C. Changes from Last Call (Informative)

This appendix provides an informative account of the changes from the
last-call version of this document.
Post-last call changes to the document, except for some minor changes to
fix formatting, etc., are indicated in the style of this appendix.

C.1 Substantive changes

This section provides information on the changes to the document that make
changes to the specification of OWL.

[10 April 2003]
In response to
http://lists.w3.org/Archives/Public/www-webont-wg/2003Apr/0046.html,
added owl:Class,
owl:Restriction,
owl:ObjectProperty,
owl:DatatypeProperty,
owl:AnnotationProperty,
owl:OntologyProperty,
owl:Ontology,
owl:AllDifferent,
owl:FunctionalProperty,
owl:InverseFunctionalProperty,
owl:SymmetricProperty, and
owl:TransitiveProperty
to CI
in Section 5.2. Some of these were
inferrable already.

[6 June 2003]
Changed the treatment of datatypes to correspond with the
substantive post-last-call fixes and changes to the treatment of
datatypes in RDF.
Changes have been made in
Section 3.1 and
Appendix A.1.

[30 June 2003]
Fixed a bug in the semantic conditions for
owl:hasValue
noticed by Jeremy Carroll, changing the conditions for the value
from a property to an individual or a data value
in Section 5.2.

[23 July 2003]
In response to a substantive post-last-call change to the RDF semantics,
changing the if-and-only-if conditions for
rdfs:subClassOf and
rdfs:subPropertyOf to only-if
conditions,
added if-and-only-if conditions for
rdfs:subClassOf,
over OWL classes, and
rdfs:subPropertyOf,
over OWL individual-valued properties and
over OWL datatype properties,
to Section 5.2.

[23 July 2003]
In response to a substantive change to the RDF syntax mapping to
triples, removing the typing triples for collections,
[applicable document unknown],
made typing of list resources optional in
Section 4.1.
Also modified an example in
Appendix B.1.

C.2 Editorial changes

This section provides information on editorial changes to the document,
i.e., changes that do not affect the specification of OWL.

[3 April 2003 and following]
This appendix was added to the document and will be kept
up-to-date.

The following table provides pointers to information about each
element of the OWL vocabulary, as well as some elements of the RDF and RDFS
vocabularies.
The first column points to the vocabulary element's major definition
in the abstract syntax of Section 2.
The second column points to the vocabulary element's major definition
in the OWL Lite abstract syntax.
The third column points to the vocabularly element's major definition
in the direct semantics of Section 3.
The fourth column points to the major piece of the translation from
the abstract syntax to triples for the vocabulary element
Section 4.
The fifth column points to the vocabularly element's major definition
in the RDFS-compatible semantics of Section 5.